1. Field of the Invention
The present invention relates to a device applied to an automatic soldering apparatus for soldering a printed circuit board which is mounted on electronic appliances, communication apparatus, measuring instruments, etc. in order to keep the quality of a flux constant and, more particularly, to a device for controlling the concentration and the temperature of a flux by adjusting the concentration, the temperature, the purity, etc. of the flux.
2. Description of the Prior Art
As one of the conditions for keeping the high quality of a soldered printed circuit board, it is generally required that good flux application has been stably achieved before soldering. Therefore, in an automatic soldering apparatus, various measures have conventionally been taken in order to keep the quality, in particular, the concentration, the purity, etc. of the flux in a flux tank constant. Such methods of controlling the concentration of a flux as those disclosed in, for example, Japanese Patent Laid-Open No. 24556/1976, Japanese Patent Laid-Open No. 11159/1974 and Japanese Patent Laid-Open No. 101464/1977 have been proposed and developed. In these methods, the specific gravity of the flux in a flux tank is first measured by an electromagnetic or optical detecting means, the measured value is corrected by the value of the temperature of the flux which has been measured by a temperature sensor or the like simultaneously with the measurement of the specific gravity value, thereby obtaining the accurate concentration of the flux, and the amounts of flux stock solution and diluent to be supplied to the flux tank are adjusted so that the concentration value obtained is constantly kept within an appropriate range.
However, such a conventional device for controlling the concentration of a flux requires various complicated structural members such as a correction arithmetic circuit for calculating the accurate concentration of a flux on the basis of the measured values of the specific gravity and the temperature of the flux, and a control circuit and a control mechanism for constantly maintaining the concentration of the flux in the flux tank in the most activated range on the basis of the calculated value of the concentration of the flux and in correspondence with the temperature change of the flux. Therefore, the cost of the entire concentration controlling device inconveniently becomes high.
The feature of the above-described conventional devices lies in that the concentration of the flux in a flux tank is controlled so as to be constant on the basis of the specific gravity value of the flux which has been corrected in consideration of the temperature value. This controlling method is based on the technical view that the characteristics of the flux mainly depend upon the value of the specific gravity value of the flux. Actually, however, since the characteristics of a flux are greatly influenced by the temperature value as well as the specific gravity value, as has already been confirmed by the present inventors, a conventional device cannot constantly keep the characteristics of a flux good, specifically, when the temperature of the flux is considerably high or low. Thus, the conventional methods of controlling the concentration of a flux are not always satisfactory.
Generally, fluxes come to have an appropriate viscosity and are activated when they have a temperature of not lower than 18.degree. C. On the other hand, if the temperature of the fluxes exceed 25.degree. C., the amount of evaporation of the diluent in the flux economically disadvantageously becomes very large. Therefore, the temperature of a flux which is suitable for good application to a printed circuit board is in the range of 18.degree. to 25.degree. C., in particular, 20.degree. C. However, no special measure has conventionally been taken in order to constantly and exactly maintain the flux at an appropriate temperature, or no device for keeping the characteristics of a flux good on the basis of the temperature control of the flux has ever existed.
Furthermore, in a conventional soldering apparatus, the concentration of the flux in a flux tank is apt to be non-uniform, which may bring about a problem of the generation of slight unevenness in application.
In addition, it is generally considered to be one of the conditions for obtaining high-quality soldered products that a flux without any impurities mixed thereinto should be stably supplied to a flux tank. Therefore, an automatic soldering apparatus has hitherto been required to be equipped with a mechanism which satisfies this condition.